The M66 is/was available as both FWD and AWD, the FWD being less common. After looking around it seems that it'd just give you closer ratios, which might not make much of a difference with the power you're planning on putting out. The M56 is going to be the easiest to find. You can pull one from a junkyard for around $125. I'd say pull '96+ as some OBD things changed from the earlier ones. Otherwise the differences are small enough that you can fabricate work-arounds.

Good luck with the build. Have you checked out turbobricks yet? Lots of performance stuff is discussed there, they might have ideas. Easiest way to get your chassis right is to draw it a few times and make sure its what you want before you start. Makes building it a lot easier if you know exactly where each piece goes, and how far off they are from where you want them to be!

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If you're not driving it "like its stolen," are you really driving?